1. Field of the Invention
The present invention concerns a process for preparing tertiary alkyl ether products which are used, in particular, as components of motor fuels. The products contain, methyl t-butyl ether, ethyl t-butyl ether, t-amyl methyl ether or t-amyl ethyl ether, or mixtures thereof, and possibly heavier tertiary alkyl ethers. According to the process, isoolefins, in particular the C.sub.4 -C.sub.7 isoolefins, of an olefinic hydrocarbon feedstock are reacted with a suitable alkanol in order to prepare the corresponding ethers. These ethers are removed together with the bottoms product of the distillation reaction system and, if necessary, they are further processed in order to prepare a motor fuel component. Unreacted alkanol is removed with the overhead product of the distillation.
2. Description of Related Art
Tertiary alkyl ethers are added to gasoline in order to improve the anti-knocking characteristics thereof and to reduce the concentration of harmful components in the exhaust gases. The oxygen-containing ether group of these compounds has been found favourably to improve the combustion process of automotive engines. Suitable alkyl tert-alkyl ethers are methyl t-butyl ether (MTBE), ethyl t-butyl ether (ETBE), t-amyl methyl ether (TAME), t-amyl ethyl ether (TAEE) and t-hexyl methyl ethers (THME), to mention a few examples. These ethers are prepared by etherification of isoolefins with monovalent aliphatic alcohols (alkanols). The reactions can be carried out in a fixed bed reactor, in a fluidized bed reactor, in a tubular reactor or in a catalytic distillation column.
The etherification reaction is an exothermic equilibrium reaction, and maximum conversion is determined by the thermodynamic equilibrium of the reaction system. To use TAME as an example, it is possible to obtain an about 90% conversion by carrying out reaction and separation in a reactive distillation column, whereas only a 65 to 70% conversion is obtainable in a fixed bed reactor.
Ion exchange resins are the most common etherification catalysts. Generally the resin used comprises a sulfonated polystyrene/divinyl benzene based cation exchange resin (sulfonated polystyrene cross-linked with divinylbenzene) having particle sizes in the range from 0.1 to 1 mm.
Two types of TAME processes have been commercially available for some time. The first one comprises fixed bed reactors, columns for product separation by distillation and a methanol separation unit. In the other process, the product distillation is replaced by a catalytic distillation unit, which substantially improves the TAME conversion, as mentioned above.
A third completely novel etherification process is described in our International Patent Application WO 93/19031. This novel process comprises a catalytic distillation unit which has been modified by transferring the catalyst conventionally placed in the distillation column into a separate external reactor which is being fed from the product separation distillation unit. The side reactor product is recycled back to the same product separation distillation unit. According to an embodiment of that process described in our international patent application WO 93/19032, the product distillation of the catalytic distillation reactor system is operated in such a way that most, and preferably practically all, of the alkanol which is removed with the distillate is bound to the inert C.sub.4 hydrocarbons of the distillate, forming an azeotrope with them. The product is recovered from the bottom of the column and it comprises a mixture of TAME and heavier ethers.
The process described in our international patent applications mentioned above can also be used for preparing lower alkyl ethers, such as methyl t-butyl ether (MTBE) and ethyl t-butyl ether (ETBE), and mixed ether products containing such ethers.
A suitable feedstock for the above-mentioned processes for preparing tertiary alkyl ethers is Fluidized Catalytic Cracking (FCC) Gasoline containing C.sub.4-7 hydrocarbons, a substantial portion, generally at least 5%, typically about 7 to 30 wt-%, of which comprises reactive C.sub.4-7 isoolefins. These reactive isoolefins include the following compounds: isobutene, 2-methyl-1-butene, 2-methyl-2-butene, 2-methyl-1-pentene, 2-methyl-2-pentene, 2,3-dimethyl-1-butene, 2,3-dimethyl-2-butene, 2-ethyl-1-butene, 2-methyl-2-hexene, 2,3-dimethyl-1-pentene, 2,3-dimethyl-2-pentene, 2,4dimethyl-1-pentene, 2-ethyl-1-pentene and 2-ethyl-2-pentene. Other suitable hydrocarbon feedstocks for etherification processes are formed by Pyrolysis C.sub.5 Gasoline, Thermofor Catalytic Cracking (TCC) Gasoline, Residual Catalytic Cracking (RCC) Gasoline and Coker Gasoline.
Although the above-mentioned novel etherification process will provide excellent conversion rates of the reactive C.sub.4 's and C.sub.5 's, the conversion of the reactive C.sub.6 's to the corresponding tertiary alkyl ethers (e.g., THME, tert-hexyl methyl ether, THEE, tert-hexyl ethyl ether) is less than 50%. Depending on the process configuration it can even be less than 40 or 30%. In a mixture containing C.sub.4, C.sub.5 and C.sub.6 -based ethers and the corresponding non-reactive hydrocarbons, an increase of the amount of C.sub.6 ethers would significantly reduce the vapor pressure of the ether products, improve the octane number thereof and, considering the fact that the alkanol is a more inexpensive component than the gasoline, increase the cost efficiency of the process.